Magnetic resonance imaging (MRI) is a non-invasive imaging technique that may be used to display detailed tissue structures in the body of a patient. The displayed tissue structure may provide valuable information assisting in the diagnosis and treatment of various medical conditions.
An MRI system may include three components or subsystems: a magnet with a main magnetic field, a gradient system, and a radiofrequency (RF) system. The main magnetic field produced by the magnet aligns the nuclear magnetization of atoms in the body of the patient. The gradient system spatially varies the main magnetic field with corresponding pairs of gradient coils, such that the position of a slice to be imaged may be precisely located. The RF system includes antennas (e.g., coils) that are used to send RF pulses (e.g., transmitter coils) and/or receive magnetic resonance (MR) signals (e.g., receiver coils). The RF pulses produce an electromagnetic field that flips or changes the alignment of the nuclear magnetization of the atoms. When the electromagnetic field is turned off, the nuclear magnetization of the atoms decays to the natural alignment of the atoms within the main magnetic field, and the atoms release excess stored energy. When the atoms release the excess stored energy, the atoms give off MR signals that are received by the receiver coils of the RF system and are used to construct image slices. The signal strength in a receiver coil depends on the volume of excitation in the coil and the distance to the object to be measured. The noise in the MR signal, however, depends largely on the size of the coil.
MRI systems may be used to diagnose various medical conditions in children (e.g., pediatric MRI). Standard RF coils originally designed for other applications may be used for pediatric MRI. For example, a knee coil for an adult patient may be used to image the entire body of a baby. Pillows and/or other filing materials may be used to help keep the child in a fixed position during imaging. Drugs for sedation may also be used to keep the child in a fixed position. These filling materials may result in an unknown or undesired geometric relationship between a coil and the body of the patient. The coil size and poor filling may result in limitations to the signal to noise ratio and thus the image quality.